1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly relates to an image forming apparatus that forms an image by means of toner.
2. Description of Related Art
Hereinafter, the structure of a conventional general image forming apparatus is described. FIG. 9 is a sectional constitutional view of the vicinity of a transfer roller 506 in a conventional image forming apparatus 500.
The image forming apparatus 500 includes an intermediate transfer belt 502, a driving roller 504, a transfer roller 506, and a separation member 508. The intermediate transfer belt 502 is wound around the driving roller 504. The driving roller 504 is rotated by a motor (not shown), whereby the intermediate transfer belt 502 is driven. The transfer roller 506 is provided so as to be opposed to the intermediate transfer belt 502.
In the image forming apparatus 500, a toner image formed on a photoreceptor (not shown) is transferred to the intermediate transfer belt 502 (primary transfer). Subsequently, the toner image transferred to the intermediate transfer belt 502 is transferred to paper passing between the intermediate transfer belt 502 and the transfer roller 506 (secondary transfer). The toner image is negatively charged. Further, the driving roller 504 is held at a ground potential, and the transfer roller 506 is held at a positive potential. The intermediate transfer belt 502 is held at a positive potential close to the ground potential. Under these conditions, the secondary transfer of the toner image is possible by an electric field generated between the intermediate transfer belt 502 and the transfer roller 506.
Incidentally, in the image forming apparatus 500, paper comes into contact with the transfer roller 506 held at the positive potential and therefore is positively charged. For this reason, the paper sticks to the intermediate transfer belt 502 through the electric field generated between the transfer roller 506 and the intermediate transfer belt 502. Thereat, in the image forming apparatus 500, the separation member 508 held at a negative potential is provided. Thereby, the paper is discharged by the separation member 508 and separated from the intermediate transfer belt 502.
However, there has been a problem with the image forming apparatus 500 in that the quality of the toner image deteriorates when the potential of the separation member 508 is lowered for reliable separation of the paper from the intermediate transfer belt 502. More specifically, when the potential of the separation member 508 is lowered, the potential difference between the transfer roller 506 and the separation member 508 increases, and therefore, a current flows from the transfer roller 506 to the separation member 508 via the paper. This causes a decrease in transfer current flowing from the transfer roller 506 to the intermediate transfer belt 502, thereby resulting inadequate transfer of the toner image. Thus, the quality of the toner image deteriorates.
It is to be noted that as a conventional image forming apparatus, there is known, for example, an image forming apparatus described in Japanese Patent Application Laid-Open No. 2003-167450. In this image forming apparatus, the time of applying a transfer bias voltage is adjusted so as to improve the paper separation property from a photosensitive drum. In the image forming apparatus described in Japanese Patent Application Laid-Open No. 2003-167450, any separation members like the separation member 508 are not used to separate paper from the photosensitive drum, and the above-described problem of deterioration in image quality does not occur.